1. Field of the Invention
Embodiments of the invention are generally in the field of marine seismic exploration. More particularly, apparatus and methods are disclosed for more efficiently and safely deploying, manipulating, and recovering payload in an unstable (marine) environment.
2. Related Art
Seismic data, long utilized in oil exploration, is increasingly being used not only for exploration, but also in production, development, and exploitation of already producing oil fields, and is typically referred to in the art as ‘exploitation seismic.’
In the marine environment, seismic data has conventionally been collected from surface vessels towing long streamers of receivers, and introducing energy with air guns towed behind the same or a separate source vessel. During the past decade, autonomous ocean bottom receivers called ‘nodes’ or ocean bottom seismometers (OBS) have been developed. Nodes contain their own power source and record seismic data passively and continuously from the time they are placed on the sea bed and started until stopped and/or retrieved.
Three dimensional seismic imaging has been common for three decades, but in recent years, as exploitation seismic has matured, the fourth dimension, time, has importantly emerged. In 4D seismic, the identical (as nearly as possible) 3D seismic programs are repeated at time intervals ranging from a few months to a few years, and those results are then compared. The differences can be and are attributed to the changes in the oil field itself as a function of production. This in turn allows the oil field production managers to better place future wells and/or manage their injectors and current production wells to maximize the exploitation of the resource.
The costs of ocean bottom recording typically significantly exceeds that of surface seismic, predominantly incurred through the placing and recovering of the ocean bottom equipment. As oil production moves to deeper and deeper waters, these costs escalate. In the case of nodes in very deep water, the nodes are placed and recovered by heavy work class remotely operated vehicles (ROVs), which are not only expensive on their own, but also require pilots, other crew, redundancy, maintenance, power, and deck equipment further requiring larger vessels, which together make these operations exceedingly expensive. Due to the expense, ocean bottom receivers are generally placed on a very course (e.g., 200 to 600 meter) grid and are shot into with a fine surface source grid. However, merely transiting a large grid with an ROV(s) and ROV equipped vessel involves substantial time and expense.
In deep water, ROVs are most often launched and recovered from surface vessels or platforms coupled with their tether management system (TMS). Together the TMS and ROV are overboarded and suspended in the water column from the surface by an umbilical. The umbilical is usually a heavy armored cable that carries power and data connections therein, connecting the ROV/TMS to the surface. When at operating depth, the ROV is disengaged from the TMS and is able to ‘fly free’ of the TMS connected by a much lighter and more flexible cable called a tether. Like the umbilical, the tether transmits power and data between the ROV and the TMS via conductors. The TMS remains suspended in the water column beneath the surface vessel or platform by way of the umbilical.
Recovering the ROV is a two step process. The ROV must return to and dock safely with its TMS, the TMS recovering slack tether in the process. Once joined, they are winched back to the surface with the umbilical. Both operations may involve substantial hazards. In the case where the TMS is suspended from a surface vessel, it is subject the same motion (in some cases amplified motion) as the surface vessel unless heave compensation is employed. Various heave compensation means are available but all are expensive and add wear and tear on the umbilical, another exceedingly expensive item.
The joined TMS and ROV are highly susceptible to damage when transiting the air/water interface until safely secured in position on the deck, predominantly due to the motion of the vessel. Together with the fact that recovering the package from great depths can itself be time consuming, minimizing the number of times the ROV must be recovered to the vessel is crucial to efficient operations. In addition, there are safety concerns for the crew during recovery operations not present when the ROV(s) remains at depth.
For ROVs engaged in deploying nodes and other OBS system components, subsea reloading of the ROV with suitable components is a desirable alternative to recovering the ROV and reloading it on the surface. Several mechanisms to permit this are in use; for example, U.S. Pat. No. 7,632,043 discloses a second device (reloader) that is loaded on a surface vessel with a replacement payload for the ROV. This device and payload are lowered through the water column to the sea bed in close proximity to the ROV. The ROV, flying free of its TMS on its tether and using fixtures and machinery it carries designed specifically for this purpose, engages with the reloader and effects an exchange of the payload from the reloader to the ROV. After the exchange, the ROV departs the reloader and continues its mission on the sea floor while the reloader is winched back to the surface and back aboard the vessel.
As disclosed, this exchange is conducted on the sea floor for a very practical reason: the reloader is stationary on the bottom and not subject to vertical motion owing to the surface vessel's heave to which it is subject during its descent/ascent. However there are both hazards and time consuming problems associated with landing this heavy machinery on the sea bottom. The sea bed contour may not be suitable to land the reloader, or there may be other expensive ocean bottom assets that must be avoided requiring the surface vessel to reposition itself and all the suspended equipment to a more suitable location. Moreover, where the bottom is soft and or mud, visibility required to engage the reloader can be obstructed for long periods of time owing to the light currents generally encountered at significant ocean depths.
In regard to productivity, the necessity of landing the reloader on the sea bottom to effect the transfer requires the surface vessel to stop and hold position on the surface. While the transfer is in progress and until concluded, all production is halted, even in the event a second ROV, which still has payload, is in use.
Moreover, “reloaders” as described here and elsewhere have their own inherent problems. Firstly, they tend to be large massive machines making them expensive, maintenance intensive, large consumers of valuable deck space, and requiring their own launch and recovery systems (LARS), while ROVs constitute a second piece of large machinery subject to all these same shortcomings and additionally large power consumers requiring even more resources aboard.
For all the foregoing reasons and others appreciated by those skilled in the art, there exists a need to affect the exchange of nodes between a surface vessel and an ROV operating at depth without the need for “reloader” machinery at all or using minimal machinery that is sufficiently light, simple, and inexpensive that a dedicated LARS is unnecessary. Furthermore, if that transfer can be accomplished in the mid-water column while the vessel, TMS, and loader are in-transit and advancing on the next deployment location, the exchange may be accomplished with no delay owning to this activity.